(Per)fluorinated polymers containing sulfonyl fluoride functional groups are known in the prior art as precursors for a class of ion exchange (per)fluorinated polymers generally referred to as “ionomers”. These ionomers are widely used as an ionically conducting material in electrochemical applications such as fuel cells, chloro-alkali cells, lithium batteries and electrodialysis, or as a solid catalyst in reactors. In these applications, the ionomer is in contact with an aqueous or polar liquid having affinity with the ionic functional groups of the ionomer, and is often made in the form of membranes or thin films to minimize its resistance to ionic transport.
Generally, to have a better efficiency in the ionomer applications, it is desirable to have a larger amount of ionic groups present in the ionomer chain. From this point of view, an important parameter used to characterize ionomers is “equivalent weight” (EW) which, as generally accepted in the art and consistently used in the present invention, refers to the weight of the polymer in acid form required to neutralize one equivalent of NaOH. Accordingly, higher EW means that there are fewer active ionic species present in the ionomer concerned, and therefore gives inferior ion exchange capability in electrochemical application. Hence, ionomers having a low EW are desirable since they give high application efficiency in theory.
However, for electrochemical applications, the use of low-EW ionomer was previously considered as unpractical. This is because, for the fluoropolymers currently known, a decrease in equivalent weight is directly associated with an increase in water retention, or swelling degree. In use, an excess water affinity of the ionomer has as a negative consequence of an excessive polymer swelling, which assumes a gelatinous state consequently losing its physical integrity. The ionomer becomes therefore mechanically unusable in all the electrochemical applications requiring a solid polymer form, such as in fuel cell devices.
Also in the applications where the ionomer is mixed with or deposited on a support material, suitable to guarantee the shape and the physical integrity of the final membrane, the ionomer must however show a sufficient physical consistency to prevent the release from the support and it must be quite insoluble in water with which it comes into contact during the use. Besides, for sulphonic fluorinated polymer suitable for making useful ionomeric membranes, the polymer/membrane must be activated before use, wherefore the chemical transformation of the precursor groups —SO2F into the corresponding hydrolyzed form is necessary. The membrane activation is carried out first by contacting it with an alkaline aqueous solution and then with an acid solution. During this transformation phase, if the ionomer has a high swelling degree, it can partially or completely dissolve in the reaction medium. At this point, it is extremely difficult to recover the ionomer and separate it from the by-products of the transformation reaction.
Thus, in the prior art, to obtain a limited hydration of the ionomer for a sufficient physical integrity, fluorinated polymers having a high EW are used at the compromise of high application efficiency. An example of said ionomer is represented by the commercial product NAFION® from Dupont, which is a sulphonic fluorinated ionomer used in fuel cells and has a high EW in the order of 1,000 to 1,200, i.e. corresponding to a low concentration of sulphonic groups. Although NAFION® membranes generally demonstrate good physical integrity at room temperature, they typically require a high thickness of at least 100 μm. Besides, if these membranes are used at temperatures higher than 100° C., the water contained in the membrane, due to the limited number of hydrophilic groups —SO3H and the high thickness, tends to diminish, wherefore the membrane tends to dehydrate and the membrane conductivity is drastically reduced. Consequently, the NAFION® membranes are not effectively usable at temperatures higher than 100° C.
In an attempt to circumvent this prior art limitation, EP 1589062 A (SOLVAY SOLEXIS S.P.A.) 26 Oct. 2005 discloses ionomer membranes comprising (per)fluorinated ionomers adapted for use in fuel cells under fully hydrated conditions and at temperatures higher than 100° C., including certain membranes formed of copolymers TFE/F2C═CF—O—(CF2)2—SO2F.
Additionally, efforts have been made in the art to deliver ionomer membranes for hydrogen-based fuel cells which are able to operate in so-called “dry conditions”, i.e. without the need of sophisticated water management systems, and/or at temperatures up to 120° C.
Within this scenario, U.S. Pat. No. 7,094,851 (GORE ENTREPRISE HOLDINGS) 22 Aug. 2006 discloses ionomers having low EW (typically between 625 and 850 g/eq) and high conductivity (greater than 0.13 S/cm), which are capable of being processed into thin film and are well-suited for low humidity or high temperature fuel cell applications. Nevertheless, ionomers hereby described comprising recurring units derived from tetrafluoroethylene (TFE) and from a comonomer of formula (A):

wherein X is F, Cl or Br or mixtures thereof; n is an integer equal to one or two; Rf and R′f are independently selected from the group of F, Cl, perfluoroalkyl radical, and chloroperfluoroalkyl radical; Y is an acid group or a functional group convertible to an acid group, like notably —SO3Z, with Z being H or any combination of cations; a is zero or an integer greater than zero; and b is an integer greater than zero, are known to possess poor temperature resistance, so that membranes prepared therefrom cannot withstand long-life fuel cell operations at temperatures exceeding 65° C.
Similarly, U.S. Pat. No. 7,041,409 (GORE ENTERPRISE HOLDINGS, INC.) 9 May 2006 discloses fluorinated ionomeric co-polymers comprising:
(a) a substantially fluorinated backbone;
(b) pendant groups derived from an ionomeric monomer of the formula (A)

wherein X is F, Cl or Br or mixtures thereof; n is an integer from zero to two; Rf and R′f are independently selected from the group of F, Cl, perfluoroalkyl radical, and chloroperfluoroalkyl radical; Y is an acid group or a functional group convertible to an acid group, like notably —SO3Z, with Z being H or any combination of cations; a is zero or an integer greater than zero; and b is an integer greater than zero; and
(c) pendant groups derived from a vinyl ether monomer that has at least two vinyl ether groups of the form, CA2=CB—O—, where the vinyl groups are separated by greater than four atoms; A is independently selected from the group containing F, Cl, and H; and B is independently selected from F, Cl, H and ORi, where Ri is a branched or straight chain alkane that may be partially, substantially or completely fluorinated or chlorinated, said copolymers being particularly well-suited for low humidity of high temperature fuel cell operations. Nevertheless, such ionomers were found to exhibit limited durability in high temperature PEMFC operations, due to excessive swelling.
Another similarly structured fluorinated co-polymer is mentioned in JP 63048314 A (NIPPON MEKTRON KK) 1 Mar. 1988, which is prepared by copolymerization of components (a)-(c) below:
(a) CF2═CF—(CF2)l—O—(CF2CF(CF3)O)m—(CF2)n—SO2F, wherein l is 0 or 1, m is an integer between 0-2, and n is 1-4;
(b) at least one fluorinated monomer selected from tetrafluoroethylene, trifluoroethylene, perfluoropropoxypropylaryl ether, and combination thereof; and
(c) at least one fluorine-containing diene monomer selected from CF2═CFOCF═CF2, CF2═CFCF2OCF2CF═CF2, CF2═CFOCF2CF═CF2, and combination thereof, wherein the component (a) is present in the copolymer at an amount of 0.1-10 wt %. The advantage of such copolymers, according to JP 63048314, is an improved thermal and chemical resistance for industrial uses. However, due to the particular structure of the fluorine-containing diene monomer(s) selected by JP63048314, they are highly unlikely to give crosslinking in the copolymerization process. Specifically, in a typical copolymerization process as above mentioned, a large majority of CF2═CFCF2OCF2CF═CF2 or CF2═CFOCF2CF═CF2 monomers would react intramolecularly and form a ring structure of oxane or tetrahydrofuran along the polymer main chain, due to thermodynamic consideration, instead of acting as a crosslinker to react intermolecularly; and the shortest diene monomer of CF2═CFOCF═CF2 mentioned above is difficult to be prepared and would eventually give a very tight crosslinking due to the closeness of two double bonds in one molecule, therefore not easily adaptable for industrial uses.
Therefore, the need was felt to have new, crosslinked fluorinated ionomers such that the membranes obtained therefrom have an optimal combination of high ionic conductivity, proper hydration and good physical integrity even in extremely thin thickness, both at room temperature and higher operation temperatures without substantially compromising the membrane physical integrity, to suit most electrochemical applications.
Furthermore, it would be desirable to have available sulfonyl fluoride polymers as precursors of low-EW ionomers, which are melt processable and have no or limited loss of volatile substances at their melt processing temperatures.
The Applicant has surprisingly and unexpectedly found fluorinated ionomers able to solve the aforementioned technical problem.